Leibstadt Nuclear Power Plant can restart power operation after an extended annual overhaul. The Swiss Federal Nuclear Safety Inspectorate (ENSI) has checked the clarification of the causes and measures derived from it for the avoidance of critical boiling conditions or dryouts. After the circumstances that led to the dryout had been identified, the supervisory authority granted the release for restarting the plant subject to conditions. Safe operation without risk to humankind or the environment is ensured.
“Safe operation of KKL is guaranteed and the plant fulfils the safety requirements of the legislative authority,” stated Ralph Schulz, Director of the Safety Analyses Division at ENSI. “Therefore, from the point of view of the supervisory authority, there is no reason why the Leibstadt Nuclear Power Plant cannot restart operation.” ENSI came to this conclusion after reviewing the application and the technical reports that KKL had submitted at the end of 2016.
As part of the study into the causes, KKL undertook a wide ranging and systematic inspection program on over 200 fuel elements to clarify which conditions led to the occurrence of the dryout.
The fuel element inspections yielded the following results:
- The dryout phenomenon has occurred since cycle 28 (2011/12).
- Dryout only occurred on (high power) fuel elements in their first year of operation inside the reactor and solely in two particular types of inflow positions.
- Dryout only occurred at the end of the cycle.
- Dryout indications were always found below the 7th and/or 8th spacer and always on the same fuel rods in the affected fuel element.
- The dryout indications always had the same fan-like shape, but differed in their dimensions. The length of the indications extended from a few millimetres up to 250 millimetres. In one case, the corrosion resulted in a leak.
- The resulting oxide layer varied in thickness. The maximum thickness was 260μm.
- The dryout findings relate to only one of the four fuel element types used in KKL.
- The dryout findings lead to the conclusion that cladding tube temperatures were under 800° Celsius and that the condition lasted over a long period with an alternating loss of the coolant film followed by rewetting with coolant.
- Dryout only occurred if the fuel elements had been operated with a fuel element power of more than 7.4 megawatts.
- Dryout only occurred when the core flow was above 95 percent.
“The study into the causes showed a clear and plausible pattern for the relevant influence quantities,” Ralph Schulz explained. From this pattern, the operator initiated measures that should prevent a renewed occurrence of dryouts. ENSI has checked these measures and assessed them as fulfilling the requirements. This means that the design and operation of the reactor core is now such that the conditions, that led to local dryout occurrences during the last cycles, no longer occur.
Power reduction to avoid future dryouts
As an operating and core design measure in the next operating cycle (up until the middle of September 2017), the core flow, i.e. the quantity of water that is pumped through the core, will be limited to a maximum 95 percent. Additionally the power per fuel element for fresh fuel elements in the first cycle will be limited to 7.0 megawatts for continuous operation or 7.2 for short-term operation. This corresponds to a power reduction for the fresh fuel elements of around 20 percent. The consequence of these measures is that the thermal power of the Leibstadt reactor will have a maximum value of 95 percent at the start of the operating cycle that will fall by the end of the cycle to around 88 percent.
The advice of the experts of the Paul Scherrer Institute PSI and Sten Lundberg Consulting SLC who were consulted by ENSI confirmed ENSI’s view that the measures of the operator are appropriate.
Additional monitoring and safety measures
If, contrary to expectations, fuel rod damage again occurs during the next operating cycle with an associated leaching of radioactive substances into the coolant, this would be immediately registered by the measuring systems.
As a condition for the approval for power operation it is specified that the Leibstadt Nuclear Power Plant must immediately shut down if an increase in radioactive waste gases is detected. These indicate fuel rod damage, irrespective of the cause of the damage. All fuel elements in the core must then be checked for leak-tightness. Normally a NPP would not be shut down due to fuel rod damage because of the low safety significance.
ENSI will also be regularly informed by Leibstadt Nuclear Power Plant of further progress in the detailed analysis of the physical mechanisms involved. Moreover in the next annual overhaul, KKL will again perform an extended inspection of the fuel elements.
There was never any danger for mankind or the environment
There are operational limits for the contamination of the coolant that must be complied with in the event of fuel rod damage. These were a long way from being breached. “All these measures ensure that the protection of mankind and the environment is ensured,” emphasised Ralph Schulz. He also pointed out that even with the dryouts, thanks to the multi-barrier system and the defense in depth provisions there was no danger.
Overhaul performed in line with requirements
Leibstadt Nuclear Power Plant was shut down on 02 August 2016 for its annual overhaul. The long shut-down can be traced back to the massively extended inspection program that was instigated at short notice for analysis of the dryout causes. Alongside the extensive fuel element inspection, the operator performed a range of maintenance work and periodic inspections. The reactor core has been newly configured. 84 spent fuel elements of the total 648 fuel elements were replaced by fresh fuel elements.
Experts from the Swiss Association for Technical Inspections and ENSI specialists checked the work on the plant after completion of the annual overhaul. Based on a number of inspections ENSI was able to reliably establish that consistent and legally compliant radiation protection was implemented by KKL during overhaul shut-down.
The checks and inspections did not uncover any finding that would call into question restarting ready for the next operating cycle or safe power operation.